Hydrocracking of hydrocarbon feedstocks is often used to convert lower value hydrocarbon fractions into higher value products, such as conversion of vacuum gas oil (VGO) feedstocks to diesel fuel and lubricants. Typical hydrocracking reaction schemes can include an initial hydrotreatment step, a hydrocracking step, and a post hydrotreatment step. After these steps, the effluent can be fractionated to separate out a desired diesel fuel and/or lubricant oil basestock.
One method of classifying lubricating oil basestocks is that used by the American Petroleum Institute (API). API Group II basestocks have a saturates content of 90 wt % or greater, a sulfur content of not more than 0.03 wt % and a VI greater than 80 but less than 120. API Group III basestocks are the same as Group II basestocks except that the VI is at least 120. A process scheme such as the one detailed above is typically suitable for production of Group II and Group III basestocks from an appropriate feed.
One way to improve the yield of a desired product is to use catalytic dewaxing to modify heavier molecules. Unfortunately, conventional methods for producing low pour point or low cloud point diesel fuel and/or lubricant oil basestock are hindered due to differing sensitivities for the catalysts involved in the various stages. This limits the selection of feeds which are potentially suitable for use in forming dewaxed diesel and/or Group II or higher basestocks. In conventional processing, the catalysts used for the hydroprocessing and hydrocracking of the oil fraction often have a relatively high tolerance for contaminants such as sulfur or nitrogen. By contrast, catalysts for catalytic dewaxing usually suffer from a low tolerance for contaminants. In particular, dewaxing catalysts that are selective for producing high yields of diesel and high yields and high VI lube oil and are intended to operate primarily by isomerization are typically quite sensitive to the amount of sulfur and/or nitrogen present in a feed. If contaminants are present, the activity, distillate selectivity and lubricating oil yield of the dewaxing catalyst will be reduced.
To accommodate the differing tolerances of the catalysts, a catalytic dewaxing step is often segregated from other hydroprocessing steps. In addition to requiring a separate reactor for the catalytic dewaxing, this segregation requires costly facilities and is inconvenient as it dictates the order of steps in the hydroprocessing sequence.